1. Field of the Invention
This invention relates to scanning force microscopy and more particularly to a microscope in which deflection of the force sensing probe is prevented by a feedback loop in which the force between the scanning force probe and sample surface is exactly canceled by a compensating force applied to the scanning force probe by a magnet. This arrangement permits the scanning force probe to be scanned over a surface without being pulled into it by attractive atomic forces. The signal from the force compensating feedback loop is used to drive the sample position in order to acquire topographical images of the sample surface at constant force and constant deflection of the scanning force probe.
2. The Prior Art
All scanning probe microscopes suffer from possible mechanical instabilities of the scanning probe, but the problem is most severe for the atomic force microscope (AFM) when it is operated with a soft force sensing cantilever. A soft cantilever gives a bigger response for a given strength of interaction between the probe and the sample and is thus capable of resolving finer detail in the AFM image. A highly schematic arrangement of an AFM is shown in FIG. 1. Referring to this figure, a soft cantilever 2 with an asperity serving as a probe 4 is held some distance y.sub.1 from the surface of a sample 6. As the sample is moved towards the probe, interaction forces cause a deflection of the cantilever. This is shown in FIG. 2 where the cantilever has been bent down an amount x in response to the surface force at a probe-surface distance y.sub.2. An image of the surface topography is formed by adjusting the gap, y.sub.2, so that the deflection of the cantilever, x, is kept constant. The adjustments in the gap, required to maintain a constant deflection x, mapped as a function of the position of the probe over the surface, generate a map of the surface topography of the sample.
The instability arises because surface forces rise rapidly as an inverse function of the gap, whereas the restoring force provided by the spring of the cantilever is linear in the displacement x. Specifically, if the surface forces vary as F(y) (where, for attractive forces, F(y) may be a/y.sup.7)) and the restoring force varies as F(x)=-kx, then an instability arises when the surface forces increase more rapidly than the restoring force as the probe moves towards the surface. That is, when dF(y)/dy&gt;k, the probe is pulled rapidly into the surface as illustrated in FIG. 3. This behavior is often observed when moving a soft cantilever towards a surface as a discontinuity in a plot of the deflection of the cantilever against the distance that the sample is moved towards the probe. This is illustrated in FIG. 4 which shows schematically the relative deflection of the cantilever as the sample is advanced towards the probe. This advance is represented by movement along the horizontal axis of FIG. 4 from right to left. The response of the probe depends upon whether it is being advanced towards the surface for the first time (curve labeled "IN") or retracted after contact with the surface (curve labeled "OUT"). Far from the surface ("A") the probe might be pulled down a little as shown at 10 by attractive interactions between the probe and surface. When surface forces increase more rapidly than the restoring force provided by the cantilever (at "B") the probe jumps to contact with the surface as shown at 12. When the probe is in contact with the surface ("B" to "C") the probe is pushed up with the surface, giving the constant increase in deflection as shown at 14. When the surface is pulled away (C.fwdarw.B.fwdarw.D.fwdarw.E), the probe rides down with it. When the original point of contact is reached ("B") the surface usually remains stuck to the probe owing to adhesive interactions. These adhesive interactions keep the probe in contact with the surface ("D") until the force generated by bending of the cantilever is big enough to overcome the adhesive interactions and pull the cantilever away ("E"). The probe then jumps away from the surface as shown at 16. Thus, once the probe has jumped into contact with the surface, a large range of interaction forces are inaccessible to and unmeasurable by the AFM. This inaccessible range corresponds to the probe deflection at the jump labeled 12 on the way in and the jump labeled 16 on the way out.
A solution to this problem has been proposed by Joyce et al. [Physical Review Letters 68, 2790, 1992] and it is illustrated in FIG. 5. A rigid cantilever 18 is pivoted about its midpoint 20 so that it can rock about midpoint 20. A force sensing probe 22 is attached to one end of cantilever 18 and interacts with a sample 24. The rocking cantilever 18 is an electrical conductor and it interacts with two other conductors 26 and 28 placed in close proximity to it. This arrangement corresponds to two electrical capacitors, one formed by the cantilever 18 and the conductor 26 above the probe 22 and the other formed by the cantilever 18 and the conductor 28 above the end of cantilever 18 opposite to the probe 22. The capacitor (elements 18, 26) above the probe 22 is used as a detector to sense movement of the probe 22. Small variations in the spacing are sensed as variations in an AC signal 27 derived from a conventional capacitance measuring bridge circuit 29. The second capacitor formed from elements 28 and 18 serves to apply an electrostatic force in just such a manner as to cancel motion of cantilever 18. This is achieved by applying a high voltage between the capacitor plate 28 and the cantilever 18. The voltage is supplied by an amplifier 21 which is fed from a DC signal obtained from a rectified output of the measuring bridge 29. The phase of the rectified signal is chosen so as to result in a negative feedback which keeps the cantilever 18 at its initial position so that no error signal is generated by the measuring bridge circuit 29. In this way, motion of cantilever 18 is prevented provided that surface forces are within the range that can be compensated for by this feedback mechanism. The feedback signal 27 serves as the measure of the interaction force between the probe 22 and sample 24 and is used to control the gap between the sample 24 and probe 22 so as to generate a topographical image of the sample surface. In this case, doing so without motion of the probe 22 and without mechanical instability.
The prior art suffers from the drawback that a mechanical rocker must be constructed and this is much more complex to fabricate than the bending cantilevers in use at present (such as those described by Drake et al. [Science 243, 1586, 1989]). Furthermore, the cantilever is electrified and cannot be operated in a conductive fluid such as water.